All the energy of the sunshine which falls upon the cylinder, both as heat and as light, is absorbed in the form of heat, and the total amount of this energy can be calculated from the increase in the temperature of the water.  The energy which heated the water would have passed onward to the surface of the earth if its path had not been obstructed by the cylinder of water; and we can be sure that the energy which entered the water and changed its temperature would ordinarily have heated an equal area of the earth’s surface; and from this, we can calculate the energy falling upon the entire surface of the earth during any one day.

Computations based upon this experiment show that the earth receives daily from the sun the equivalent of 341,000,000,000 horse power—­an amount inconceivable to the human mind.

Professor Young gives a striking picture of what this energy of the sun could do.  A solid column of ice 93,000,000 miles long and 2-1/4 miles in diameter could be melted in a single second if the sun could concentrate its entire power on the ice.

While the amount of energy received daily from the sun by the earth is actually enormous, it is small in comparison with the whole amount given out by the sun to the numerous heavenly bodies which make up the universe.  In fact, of the entire outflow of heat and light, the earth receives only one part in two thousand million, and this is a very small portion indeed.

139.  How Light and Heat Travel from the Sun to Us.  Astronomers tell us that the sun—­the chief source of heat and light—­is 93,000,000 miles away from us; that is, so far distant that the fastest express train would require about 176 years to reach the sun.  How do heat and light travel through this vast abyss of space?

[Illustration:  FIG. 90.—­Waves formed by a pebble.]

A quiet pool and a pebble will help to make it clear to us.  If we throw a pebble into a quiet pool (Fig. 90), waves or ripples form and spread out in all directions, gradually dying out as they become more and more distant from the pebble.  It is a strange fact that while we see the ripple moving farther and farther away, the particles of water are themselves not moving outward and away, but are merely bobbing up and down, or are vibrating.  If you wish to be sure of this, throw the pebble near a spot where a chip lies quiet on the smooth pond.  After the waves form, the chip rides up and down with the water, but does not move outward; if the water itself were moving outward, it would carry the chip with it, but the water has no forward motion, and hence the chip assumes the only motion possessed by the water, that is, an up-and-down motion.  Perhaps a more simple illustration is the appearance of a wheat field or a lawn on a windy day; the wind sweeps over the grass, producing in the grass a wave like the water waves of the ocean, but the blades of grass do not move from their accustomed place in the ground, held fast as they are by their roots.